Thanks, so did I
It does raise one further question - link ed in a way to recent discussions about OHLE clearance. If the feeder wire is out of phase with the catenary wire in order to allow the autotransformer to convert power locally.... then surely there's a 70kv peak differential (50kv is average differential across the wave) between 2 wires that could be within a metre of each other.
Is the risk of arcing actually much less than we've historically supposed?
Arcing through dry air is not a problem for 2 x 25kV lines 1 metre apart. I seem to remember 10kV per inch was the rule of thumb in my youth. If you look at the OHLE anywhere you will find relatively small air gaps between 25kV and earthed metal structures (ie less than a metre). If you stand on any of the platforms on the core of Thameslink the clearance is surprisingly small between the conductor wire and the vehicle bodies.
Spark gaps are used as surge protectors on overhead lines (ie local distribution lines) and they are only a few inches or so gap. These are to spark and therefore dump to earth a lightning strike (>>100kV) thereby protecting the equipment on the end of the line.
There was a nasty incident at St Pancras a few years ago where a faulty (incorrectly set) spark gap allowed a live broken conductor wire to dance around on the platform while passengers were getting off a Eurostar train so they are used on those trains as well.
However there is always the problem of rain, foreign bodies and plain bad luck. One issue for the railway is there are a lot of clearances which move. The track moves (slowly), the OHLE moves, the trees next to the line grow and move when windy, the trains move on their suspension etc etc. So static clearances have to be set generously.
In the case of bad luck I remember many years ago around Christmas a rogue 20 ft length of tinsel floating down into the station allowed a discharge between the OHLE and the inside of an signalling equipment cabinet undergoing maintenance which led to a considerable amount of disruption while the frazzled bits were replaced!
